1. Technical Field
The present invention relates generally to testing systems for testing the electrical continuity and/or integrity of conductive line segments on circuit boards; and, more particularly, to improved capacitance circuit board testing systems characterized by their simplicity, reliabilty, relatively high precision accurate capacitance readings, economy, and by their ability to permit rapid capacitance testing for n test points (where "n" may commonly comprise on the order of 500 or more line segment end points on each different circuit board), and by their ease and rapidity of removal of one board following testing and insertion of the next board for testing. To this end, the present invention provides for the use of a circuit board backside reference plane formed of conductive elastomeric material which readily conforms in shape to the backside surface configuration of the board undergoing test irrespective of board warpage, curvature and/or other unintentional irregularities and/or intentional design irregularities, thereby enabling the circuit board to be securely clamped in place with the backside surface of the board and the conductive elastomeric material in mutually coextensive face-to-face relation devoid of critical air gaps by any suitable board mounting means such, for example, as a vacuum system for urging the board to be tested against the conductive elastomeric reference plane.
2. Background Art
The need for an effective system for verifying the electrical continuity and integrity of line segments and/or circuits in circuit boards has perplexed the industry for decades. Many highly sophisticated and advanced types of circuit boards have been developed over that period of time which, unfortunately, suffer from severe production constraints imposed by known quality control techniques. For example, industry has long known how to make complex multi-layer circuit boards on a rapid mass-production basis; but, in order to sell such circuit boards, each must be separately tested prior to shipment. Such boards will commonly have many hundreds of line segments disposed in various layers, each of which must be tested for opens and/or shorts. The most reliable known way of accurately detecting the presence of an open and/or short has been by measuring the resistance of each line end point against all other line end points. However, resistance test measurements require approximately n.sup.2 /2 separate tests and readings for a given circuit board having n test points--e.g., n line end points--and, consequently, a board having 500 or more test points (a common and often encountered situation) requires that approximately 125,000, or more, separate resistance measurements be made. This requirement has led to the development of numerous highly expensive and complex test stands having multiple test probes which are designed for use with specific types and designs of circuit boards; and, such test equipment has proven extremely expensive, difficult to use, and, further, has been characterized by its unreliability. Moreover, the equipment is space-consuming and often characterized by its lack of versatility or universal test applicability in the manufacturing facilities of circuit board manufacturers who commonly manufacture thousands of different types of circuit boards intended for numerous different customers in different industrial applications.
The prior art is replete with representative patents illustrating the types of test fixtures that have been developed for testing electrical continuity and/or integrity of line segments in printed circuit boards. For example, U.S. Pat. Nos. 4,017,793--Haines, 4,061,969--Dean, 4,115,735--Stanford, 4,164,704--Kato et al, 4,209,745--Hines, 4,321,533--Matrone and 4,322,682--Schadwill are illustrative of printed circuit board testing heads employing a plurality of test probes generally disposed in X-Y coordinate axes arrays defining a "bed of nails" approach wherein provision is made for supporting the boards to be tested on a support enabling the underlying probes to be brought into electrical contact with the multiple test points on the circuit board. In the aforesaid Haines patent, the support comprises an elastomeric resilient foam pad; Matrone discloses the use of interchangeable plates defined as "card personalizers" intended to increase the universal applicability of the system; while Schadwill discloses a vacuum testing head. U.S. Pat. No. 4,056,773--Sullivan discloses a flexible multi-point test fixture; while U.S. Pat. No. 3,830,956--Wootton et al is representative of special printed circuit board designs incorporating test pads to facilitate testing. Other illustrative types of circuit board testing devices include those disclosed in U.S. Pat. Nos. 2,844,250--Bayha et al, 3,992,663--Seddick, and 4,186,338--Fichtenbaum.
The industry has long recognized that capacitance-type testing devices--as opposed to resistance measuring devices--represent an opportunity to significantly speed up the time required for testing electrical continuity and/or integrity in printed circuit boards since such capacitance measuring devices require only n measurements to test a board having n test points--albeit at a potential tradeoff of test accuracy and reliability since capacitance measuring systems provide only an indirect indication of the presence of shorts and/or opens; and, therefore, precise accurate capacitance readings are essential in any such device in order that it be viable for use in production test work. An early example of this type of testing system is that disclosed in an article authored by Robert W. Wedwick entitled "Continuity Testing By Capacitance", CIRCUITS MANUFACTURE, November 1974, pp. 60 and 61. In this device, measurements are made of the line capacitance in a circuit board between each line segment and an internal reference plane such as a ground or power plane; and, by then comparing measured capacitance values with the known accepted values previously determined for that particular line segment. The device employs two probes, one of which is attached to the internal ground or power plane and the other of which is sequentially engaged with the line end points to be tested. While this type of test device has proven satisfactory in use, it has found only limited application for use with those specific circuit boards containing internal ground or power planes which define a reference plane to which capacitance measurements can be referenced.
U.S. Pat. Nos. 3,975,680--Webb and 4,229,693--Irick et al are directed to capacitance testing devices which are intended to improve the versatility of the capacitance test procedure; and, in the case of the Irick et al patent, to render the capacitance test procedure suitable for use with circuit boards which do not include internal reference planes. Thus, Webb discloses a capacitance testing device which includes a superimposed coupling plate which defines, in effect, a "floating reference plane" disposed above, in close proximity to and, hopefully, coplanar with the circuit board to be tested. The coupling plate is electrically connected to one terminal of a capacitance meter and a probe connected to a second terminal on the meter is inserted through an opening in the floating reference plane and sequentially applied to the line end points on the board undergoing test. Aside from the physical disadvantages of this system in terms of loading and/or unloading circuit boards into and/or from the test fixture and physical movement of the probe and/or board relative to one another during sequential application of the probe to different line end points, other severe disadvantages are inherently imposed by the system. Thus, as previously indicated, in order that individual capacitance measurements can be employed in testing circuit boards, it is essential that the system be capable of attaining highly accurate precise measurements. Unfortunately, however, the floating reference plane system interposes an air gap between the reference plane and the board undergoing test; and, any variations in the air gap create variations in the capacitance measurements which result in erroneous readings. Moreover, variations in the air gap are inherent in this type of system for many reasons including, for example, warpage and/or curvature of the circuit board and/or the floating reference plane, as well as the physical configuration of the circuit board itself.
Apparently recognizing the foregoing limitations, Irick et al have developed a somewhat more complex capacitance testing system comprising an enclosed vacuum chamber into which each circuit board to be tested must be inserted, the chamber closed and sealed, and thereafter measuring capacitance between a backside reference plane defined by the base of the vacuum chamber and line segments by inserting a pointed probe through a thin plastic membrane defining the upper wall of the vacuum chamber and into electrical contact with the line ends for the particular line segment to be tested. Thus, not only must each circuit board be individually inserted into and removed from the enclosed vacuum chamber--a time-consuming operation--but, moreover, the very test procedure employed requires puncturing or penetrating the thin membrane defining the vacuum chamber wall n times in order to make n capacitance measurements; thereby severely denegrating the reliability of the vacuum system. Moreover, in order to make the requisite electrical contact, it is essential that the probe actually puncture the plastic membrane; and, not only has it been difficult to achieve such penetration on a consistant basis, but, in addition, it is extremely difficult to ascertain with certainty that penetration has, in fact, occurred and that electrical contact has actually been made.
Another patent of interest is U.S. Pat. No. 3,243,701--Strand which discloses a ballistic galvanometer circuit for measuring capacitance to determine the thickness of insulating coatings. In this device, the patentee employs a resilient current conducting probe which is applied to the nonconductive insulating coating on a conductive substrate with capacitance measurements providing an indication of the thickness of the nonconductive coating.
Kattner et al, U.S. Pat. No. 3,405,361 discloses a fluid actuable multi-point microprobe for testing semi-conductor chips wherein the test device employs a vacuum chuck for holding the semi-conductor chip in place.
In U.S. Pat. No. 3,596,228--Read, Jr., et al, the patentees apply hydrostatic pressure through a flexible dielectric membrane to contacts on integrated semi-conductor chips. The membrane includes a conductive elastomer which serves as a ground plane in the high frequency measurements.
An article written by L. W. Holmstrom entitled "Electrical Fixture For Printed-Circuit Assemblies", IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 20, No. 8, p. 2934, January, 1978, discloses the use of a conductive elastomer to provide a compressionable medium which allows for matching irregular printed-circuit board assemblies to a fixed head plate. The conductive elastomer must include wires embedded in nonconductive material to achieve contact between terminals placed on top of each other without shorting out the printed circuit boards.
However, notwithstanding the extensive research and development work as reflected by the foregoing representative prior art disclosures, prior to the advent of the present invention there has been no effective, reliable and practical capacitance testing system for circuit boards suitable for use in manufacturing environments on circuit boards produced on a mass-production basis. Thus, the aforesaid Wedwick capacitance testing device has seen only limited application in systems suitable for testing circuit boards having internal power and/or ground planes; while capacitance measuring systems such as those disclosed in the aforesaid U.S. Pat. Nos. 3,975,680--Webb and 4,229,693--Irick et al generally find only limited application because of precise control requirements with respect to air gaps between a floating reference plane and the circuit board undergoing test, the use of plastic sheets to define an enclosed vacuum system wherein the sheets must be punctured or penetrated to establish line segment/probe contact with consequent denegration of the vacuum system, the complexities involved in loading and unloading the test fixture with successive circuit boards, and/or relative maneuverability of the test probe with respect to the n line ends on the circuit boards. The present invention overcomes all of the foregoing disadvantages and, for the first time, provides a capacitance measuring system which enables rapid, precise capacitance measurements on a continuous, easily replicated basis; while at the same time, facilitating loading and/or unloading of the test fixture and readily accommodating various known automating techniques.